This patent application is related to the following commonly owned, U.S. utility patent application Ser. No. 09/098,631 entitled xe2x80x9cRapid Signal Acquisition and Synchronization For Access Transmissions,xe2x80x9d filed Jun. 16, 1998, now U.S. Pat. No. 6,044,074 which is incorporated herein by reference.
I. Field of the Invention
The present invention relates generally to the field of wireless communications. More particularly, the present invention relates to resolving frequency and timing uncertainty in access channel transmissions in a spread spectrum communication system.
II. Related Art
Typical wireless satellite-based communications systems include base stations referred to as gateways, and one or more satellites to relay communications signals between the gateways and one or more user terminals. Gateways provide communication links for connecting a user terminal to other user terminals or users of other communications systems, such as a public telephone switching network. User terminals can be fixed or mobile, such as a mobile or portable telephone. They may be located near or remote from a gateway.
Some satellite communications systems employ code division multiple access (CDMA) spread-spectrum signals, such as disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,xe2x80x9d and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1998, entitled xe2x80x9cMethod and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy,xe2x80x9d both of which are assigned to the assignee of the present invention, and are incorporated herein by reference.
In satellite communication systems employing CDMA, separate communication links are used to transmit communication signals, including paging, access, messaging, or traffic signals, to and from a gateway or base station. A forward communication link refers to communication signals originating at a gateway or base station and transmitted to a user terminal. A reverse communication link refers to communication signals originating at a user terminal and transmitted to a gateway or base station.
The reverse link is comprised of at least two separate channels: an access channel and a reverse traffic channel. The access channel is used by a user terminal to xe2x80x9caccessxe2x80x9d a gateway. A user terminal accesses a gateway to register with the system, to place a call, or to acknowledge a paging request sent by the gateway. A user terminal communicates with a gateway on the access channel by transmitting a signal referred to as an xe2x80x9caccess probexe2x80x9d to the gateway. An access probe is a transmission of data on the access channel that contains an access message. The contents of the access message depend on whether the user terminal is initiating a call, registering with the system, or responding to a page.
In a typical spread spectrum communications system, one or more preselected pseudo noise (PN) code sequences are used to xe2x80x9cspreadxe2x80x9d information signals, such as an access probe, over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communications signals. PN code spreading, a method of spread spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal.
In order for a gateway to acquire an access probe sent by a user terminal (i.e., recover the access message within the access probe), the gateway must first demodulate the communication signal to recover the PN modulated access probe, and then despread the message portion of the access probe. In order for the gateway to demodulate the carrier, the gateway must be tuned to the carrier frequency of the communication signal. Without reasonably accurate frequency tuning, the carrier cannot be properly demodulated. Furthermore, because PN spreading codes are applied to the access probe, the arrival time of the access probe must be determined to properly despread the access probe to recover the information contained therein. PN spreading codes cannot be accurately removed without appropriate system timing or signal synchronization. If the codes are applied with incorrect time synchronization, the communication signals will simply appear as noise and no information is conveyed.
Communication systems employing satellites with non-geostationary orbits exhibit a high degree of relative user terminal and satellite motion. The relative motion creates fairly substantial Doppler components or shifts in the carrier frequency of signals within the communication links. Because these Doppler components vary with user terminal and satellite motion, they create a range of uncertainty in the frequency of the carrier signal, or more simply, frequency uncertainty. Similar effects may be observed in terrestrial systems where the user terminal is moving at a high speed, such as when used on a high speed train or other vehicle.
The satellite motion also introduces Doppler into the PN spreading codes. This Doppler is referred to as code Doppler. In particular, code Doppler is the effect of the satellite motion introduced into the baseband signal. Code Doppler shifts the frequency of the transitions between adjacent codes in the PN spreading code sequences. Thus, the adjacent codes do not arrive at the receiver with a correct code timing.
In addition to code Doppler, the satellite motion also creates a large amount of uncertainty in the propagation delay, or timing uncertainty, for signals within the communication links. For signals arriving at the gateway, the propagation delay varies from a minimum when the satellite is directly overhead of the gateway to a maximum when the satellite is at a horizon with respect to the gateway.
As stated above, in order for the gateway to acquire an access probe, the gateway must be tuned to the carrier frequency of the communication signal and synchronize timing with the signal. One way to tune the gateway to the carrier frequency and synchronize timing is to determine the carrier frequency and timing prior to the transmission of the communication signal and then tune the gateway appropriately. But because of the frequency and time uncertainty introduced into the communication signal by the Doppler effect and propagation delay, a gateway cannot determine the carrier frequency or signal arrival time prior to receiving the signal. Nevertheless, the gateway can determine the range of possible carrier frequencies and the range of possible arrival times by determining the amount of uncertainty introduced by the Doppler effect and propagation delay. Consequently, a gateway can acquire an access probe by xe2x80x9csearchingxe2x80x9d for the correct frequency and timing by comparing the received communication signal with various frequency and timing values within their respective possible ranges.
These various frequency and timing values are termed frequency and timing hypotheses, respectively. The frequency and time hypothesis with the highest correlation to the received communication signal above a predetermined threshold provides frequency and timing values that can be used to demodulate and despread the signal, thereby enabling the gateway to recover the information within the access probe.
The amount of hardware that is required to xe2x80x9csearchxe2x80x9d for the correct frequency and timing in a fixed amount of time is proportional to the number of required hypotheses, and the number of required hypotheses is a function of the range of time and frequency uncertainty. Because searcher hardware is expensive and because it is undesirable to increase the search time, a system and method to reduce the range of time and frequency uncertainty is therefore desired.
The present invention is directed toward acquiring a signal in a communication system that experiences Doppler and propagation delay due to relative motion of satellite repeaters and user terminals. Doppler effects and propagation delays introduce wide ranges of frequency uncertainty and timing uncertainty in the signals transmitted between the user terminals and the satellites and signals transmitted the satellites and the gateways. The present invention is aimed at reducing the range of frequency and timing uncertainty in the communication system. The present invention reduces the range of frequency and timing uncertainty by determining the frequency and time uncertainties over individual satellite beams rather than over an entire satellite footprint.
In one aspect the invention provides a method for acquiring a signal transmitted by a user terminal to a satellite and relayed by the satellite to a gateway. The method includes the steps of: (1) defining an arrival time and frequency search space for a communication beam associated with the satellite based on a predetermined beam coverage area of the communication beam; (2) searching the search space to resolve a timing and frequency uncertainty associated with the signal; and (3) demodulating a message portion of the signal based on a frequency increment and timing offset obtained as a result of resolving the timing and frequency uncertainty.
Preferably, the predetermined coverage region of the communication beam corresponds to an area defined by a range of azimuths and a range of elevations containing the nominal coverage region of the beam.
Advantageously, the signal transmitted by the user terminal includes a preamble portion as well as the message portion. In one embodiment, the preamble portion contains null data. Preferably, the preamble portion has a first stage modulated by a first signal and a second stage modulated by the first signal and a second signal. In one embodiment, the first signal and the second signal are pseudonoise (PN) code pairs.
According to one embodiment, the step of searching the search space includes the steps of: (1) performing a coarse search of the search space to resolve a frequency uncertainty associated with the signal; and (2) performing a fine search to resolve a timing uncertainty associated with the signal.
Preferably, the search space is defined by a range of frequencies and a range of arrival times.
In another aspect the present invention provides a method for recovering at a gateway information within a message portion of a signal transmitted by a user terminal and relayed by a satellite to the gateway. The method includes the steps of: (1) assigning an access channel receiver within the gateway to a beam associated with the satellite; (2) assigning a search space to the access channel receiver, where the search space corresponds to a frequency and timing uncertainty associated with the beam to which the access channel receiver is assigned; (3) searching the search space to acquire the signal; and (4) if the signal is acquired after searching the search space, demodulating the message portion of the signal to recover the information contained therein.
The invention also provides a system for recovering at a gateway information within a message portion of a signal transmitted by a user terminal to a satellite and relayed by the satellite to the gateway. The system includes an access channel receiver within the gateway that is assigned to a beam associated with the satellite. The system also includes a search space that is assigned to the access channel receiver. The search space corresponds to a frequency and timing uncertainty associated with the beam to which the access channel receiver is assigned. Lastly, the system includes a gateway demodulator for searching the search space to acquire the signal and for demodulating the message portion of the acquired signal to recover the information contained therein.
Preferably, the gateway demodulator includes means for performing a coarse search of the search space to resolve a frequency uncertainty associated with the signal and means for performing a fine search to resolve a timing uncertainty associated with the signal.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.